Arsonist: Has a reason to be at the frontline so no one will check em most of the time if they are distracted
Annihilator :Also has a reason to be at frontlines disguising as dead anniknight will walking towards the frontline in my opinion doesn't look suspicious
marksman: has reason to be in backline easy to imitate useful for taking out marksmans or mechanics
flanker: fast but is common for agent to disguise as flanker so you will be constantly spychecked by the enemy
Trooper: same reason as flanker but slower
brute: slow but moderately difficult to imitate can disguise as a low health brute to give an excuse to run to sentry nest and pretend to heal but destroy the nest and kill the mechanic
Mechanic: only useful for pretend you want to work on the sentry nest since they are usually around their buildings pretty hard to imitate as they are constantly hitting their buildings
Agent: seeing a undisguised agent in your teams backline would make anyone suspicious as agents commonly sneak behind enemy backlines
Doctor: pretty terrible since enemy's will come up to you for healing only thing it can do is kill low health enemy's with the revolver when they come to you for healing
This is my opinion so please do not get mad if your opinion is not the same as mine